When you start your computer up after an improper shutdown/system crash, it will usually suggest acti*** like booting into safe mode and running a disk check. How does the computer know it crashed?

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问题

超级用户读者FrozenKing想知道他的电脑是如何知道什么时候死机或是不正常关机的。他写道：

Actually, this question struck me because of power cuts in my house. When there is a power cut, there is a sudden loss of power from the computer.

How does the computer know that the shutdown was not properly done?

这是一个有趣的问题；很明显，现代操作系统很清楚地意识到出了问题，但它们是通过什么机制意识到的呢？

答案

超级用户贡献者David Schwartz对不同的操作系统提供了一般性的答案和见解：

For non-Windows based PCs, the detection is usually done on a per-filesystem basis. When a filesystem is mounted in read/write mode, an entry is written to the filesystem marking it dirty. When the filesystem is unmounted, an entry is written marking it clean. On startup, the operating system checks if its core filesystems are marked dirty, and if so it knows there wasn’t a clean shutdown — at least of those filesystems.

另一个贡献者ChrisF更具体地回答了Windows操作系统：

Windows also uses the Dirty Bit method to detect whether the PC was shut down properly:

When powered off as normal, the bit is switched off. However, in case of power outage or improper (forced) shutdown, the bit will still be on the next time the PC is started.

This means that Windows can suggest remedial action – such as booting into Safe Mode.

In addition a Windows based PC will write an entry into event log detailing when and why (if known) it was shut down. It also writes an event when the PC starts up.

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